Cation effects on protein conformation and transport in the Na+/glucose cotransporter

Functional characterization of SGLT1 using SSM-based electrophysiology: Kinetics of sugar binding and translocation

Beside the ongoing efforts to determine structural information, detailed functional studies on transporters are essential to entirely understand the underlying transport mechanisms. We recently found that solid supported membrane-based electrophysiology (SSME) enables the measurement of both sugar binding and transport in the Na⁺/sugar cotransporter SGLT1 (Bazzone et al, 2022a). Here, we continued with a detailed kinetic characterization of SGLT1 using SSME, determining K_M and K_D ^app for different sugars, k_obs values for sugar-induced conformational transitions and the effects of Na⁺, Li⁺, H⁺ and Cl^- on sugar binding and transport. We found that the sugar-induced pre-steady-state (PSS) charge translocation varies with the bound ion (Na⁺, Li⁺, H⁺ or Cl^-), but not with the sugar species, indicating that the conformational state upon sugar binding depends on the ion. Rate constants for the sugar-induced conformational transitions upon binding to the Na⁺-bound carrier range from 208 s^-1 for D-glucose to 95 s^-1 for 3-OMG. In the absence of Na⁺, rate constants are decreased, but all sugars bind to the empty carrier. From the steady-state transport current, we found a sequence for sugar specificity (V_max/K_M): D-glucose > MDG > D-galactose > 3-OMG > D-xylose. While K_M differs 160-fold across tested substrates and plays a major role in substrate specificity, V_max only varies by a factor of 1.9. Interestingly, D-glucose has the lowest V_max across all tested substrates, indicating a rate limiting step in the sugar translocation pathway following the fast sugar-induced electrogenic conformational transition. SGLT1 specificity for D-glucose is achieved by optimizing two ratios: the sugar affinity of the empty carrier for D-glucose is similarly low as for all tested sugars (K_D,K ^app = 210 mM). Affinity for D-glucose increases 14-fold (K_D,Na ^app = 15 mM) in the presence of sodium as a result of cooperativity. Apparent affinity for D-glucose during transport increases 8-fold (K_M = 1.9 mM) compared to K_D,Na ^app due to optimized kinetics. In contrast, K_M and K_D ^app values for 3-OMG and D-xylose are of similar magnitude. Based on our findings we propose an 11-state kinetic model, introducing a random binding order and intermediate states corresponding to the electrogenic transitions detected via SSME upon substrate binding.

Random-walk model of the sodium-glucose transporter SGLT2 with stochastic steps and inhibition

Random-walk models are frequently used to model distinct natural phenomena such as diffusion processes, stock-market fluctuations, and biological systems. Here, we present a random-walk model to describe the dynamics of glucose uptake by the sodium-glucose transporter of type 2, SGLT2. Our starting point is the canonical alternating-access model, which suggests the existence of six states for the transport cycle. We propose the inclusion of two new states to this canonical model. The first state is added to implement the recent discovery that the Na⁺ion can exit before the sugar is released into the proximal tubule epithelial cells. The resulting model is a seven-state mechanism with stochastic steps. Then we determined the transition probabilities between these seven states and used them to write a set of master equations to describe the time evolution of the system. We showed that our model converges to the expected equilibrium configuration and that the binding of Na⁺and glucose to SGLT2 in the inward-facing conformation must be necessarily unordered. After that, we added another state to implement inhibition in the model. Our results reproduce the experimental dependence of glucose uptake on the inhibitor concentration and they reveal that the inhibitors act by decreasing the number of available SGLT2s, which increases the chances of glucose escaping reabsorption.

PPE51 Is Involved in the Uptake of Disaccharides by Mycobacterium tuberculosis

We have recently found that selected thio-disaccharides possess bactericidal effects against Mycobacterium tuberculosis but not against Escherichia coli or Staphylococcus aureus. Here, we selected spontaneous mutants displaying resistance against the investigated thio-glycoside. According to next-generation sequencing, four of six analyzed mutants which were resistant to high concentrations of the tested chemical carried nonsynonymous mutations in the gene encoding the PPE51 protein. The complementation of these mutants with an intact ppe51 gene returned their sensitivity to the wild-type level. The uptake of tritiated thio-glycoside was significantly more abundant in wild-type Mycobacterium tuberculosis compared to the strain carrying the mutated ppe51 gene. The ppe51 mutations or CRISPR-Cas9-mediated downregulation of PPE51 expression affected the growth of mutant strains on minimal media supplemented with disaccharides (maltose or lactose) but not with glycerol or glucose as the sole carbon and energy source. Taking the above into account, we postulate that PPE51 participates in the uptake of disaccharides by tubercle bacilli.

Characterization of the transport activity of SGLT2/MAP17, the renal low-affinity Na+-glucose cotransporter

The cotransporter SGLT2 is responsible for 90% of renal glucose reabsorption, and we recently showed that MAP17 appears to work as a required β-subunit. We report in the present study a detailed functional characterization of human SGLT2 in coexpression with human MAP17 in Xenopus laevis oocytes. Addition of external glucose generates a large inward current in the presence of Na, confirming an electrogenic transport mechanism. At a membrane potential of -50 mV, SGLT2 affinity constants for glucose and Na are 3.4 ± 0.4 and 18 ± 6 mM, respectively. The change in the reversal potential of the cotransport current as a function of external glucose concentration clearly confirms a 1:1 Na-to-glucose transport stoichiometry. SGLT2 is selective for glucose and α-methylglucose but also transports, to a lesser extent, galactose and 3-O-methylglucose. SGLT2 can be inhibited in a competitive manner by phlorizin (K_i = 31 ± 4 nM) and by dapagliflozin (K_i = 0.75 ± 0.3 nM). Similarly to SGLT1, SGLT2 can be activated by Na, Li, and protons. Pre-steady-state currents for SGLT2 do exist but are small in amplitude and relatively fast (a time constant of ~2 ms). The leak current defined as the phlorizin-sensitive current in the absence of substrate was extremely small in the case of SGLT2. In summary, in comparison with SGLT1, SGLT2 has a lower affinity for glucose, a transport stoichiometry of 1:1, very small pre-steady-state and leak currents, a 10-fold higher affinity for phlorizin, and an affinity for dapagliflozin in the subnanomolar range.

Functional characterization of a novel disaccharide transporter in lobster hepatopancreas

In animals, the accepted model of carbohydrate digestion and absorption involves reduction of disaccharides into the monosaccharides glucose, fructose, and galactose followed by their individual transmembrane transport into cells. In 2011, a gene for a distinct disaccharide sucrose transporter (SCRT) was found in Drosophila melanogaster and characterized in a yeast expression system. The purpose of the present investigation was to functionally identify and characterize a putative disaccharide transporter analog in the hepatopancreas of the American lobster, Homarus americanus. Purified hepatopancreatic brush-border membrane vesicles (BBMV) were used in transport experiments using ¹⁴C-sucrose and a Millipore filter isolation technique. In the absence of sodium, an external pH of 4 significantly stimulated the uptake of ¹⁴C-sucrose compared to that occurring at pH 5, 6, or 7. At pH 7, increasing external concentrations of sodium increased ¹⁴C-sucrose uptake by BBMV in a hyperbolic fashion and this stimulation was significantly reduced when the pH was changed to 4, suggesting that both protons and sodium ions were each capable of driving the uptake of the sugar. In experiments with a variety of monosaccharides, disaccharides, and trisaccharides, used as potential inhibitors of ¹⁴C-sucrose uptake, only maltose and trehalose inhibited carrier-mediated ¹⁴C-sucrose transport. An additional experiment showed that 20 mM maltose was a competitive inhibitor of ¹⁴C-sucrose uptake. The use of a putative lobster SCRT by both maltose and trehalose is nutritionally appropriate for lobsters as they commonly digest glycogen and chitin, polymers of maltose and trehalose, respectively. These findings suggest there is a brush-border proton- or sodium-dependent, hepatopancreatic carrier process, shared by sucrose, maltose, and trehalose, that may function to absorb disaccharides that are produced from digestion of naturally occurring dietary constituents.

Changes in growth rate and thyroid- and sex-hormones blood levels in rats under sub-chronic lithium treatment

Lithium therapy, mainly used in curing some psychiatric diseases, is responsible for numerous undesirable side effects. The present study is a contribution to the understanding of the pathophysiological mechanisms underlying lithium toxicity. Male and female mature rats were divided into three batches and fed commercial pellets: one batch was the control and the second and third batches were given 2 g (Li1) and 4 g (Li2) of lithium carbonate/kg of food/day, respectively. After 7, 14, 21 and 28 days, serum levels of free tri-iodothyronine (FT3), thyroxine (FT4), testosterone and estradiol were measured. Attention was also paid to growth rate and a histological examination of testes or vaginal mucosa was carried out.

In treated rats, a dose-dependent loss of appetite and a decrease in growth rate were observed, together with symptoms of polydypsia, polyuria and diarrhea. Lithium serum concentrations increased from 0.44 mM (day 7) to 1.34 mM (day 28) in Li1 rats and from 0.66 to 1.45 mM (day 14) in Li2 rats. Li2 treatment induced a high mortality after 14 days, reaching 50-60% in female and male animals. From these data, the LD50 (14 days Li2 chronic treatment) was calculated to be about 0.3 g/day per kilogram of animal, leading to Li serum concentrations of about 1.4 mM.

A significant decrease of FT3 and FT4 was observed in treated rats. This effect appeared immediately for the highest dose and was more pronounced for FT3, resulting in an increase of the FT4/FT3 ratio. In males, testosterone decreased and spermatogenesis was stopped. Conversely, in females, estradiol increased in a dose-dependent manner as the animals were blocked in the diestrus phase at day 28. This finding supports a possible antagonistic effect of lithium on the estradiol receptors.

The role of transporter ectodomains in drug recognition and binding: phlorizin and the sodium–glucose cotransporter

This article reviews the role of segments of SLCs located outside the plasma membrane bilayer (ectodomains) using the inhibition of SGLTs (SLC5 family) by the aromatic glucoside phlorizin as a model system.

Cation-Specific Conformations in a Dual-Function Ion-Pumping Microbial Rhodopsin

A recently discovered rhodopsin ion pump (DeNaR, also known as KR2) in the marine bacterium Dokdonia eikasta uses light to pump protons or sodium ions from the cell depending on the ionic composition of the medium. In cells suspended in a KCl solution, DeNaR functions as a light-driven proton pump, whereas in a NaCl solution, DeNaR conducts light-driven sodium ion pumping, a novel activity within the rhodopsin family. These two distinct functions raise the questions of whether the conformations of the protein differ in the presence of K(+) or Na(+) and whether the helical movements that result in the canonical E → C conformational change in other microbial rhodopsins are conserved in DeNaR. Visible absorption maxima of DeNaR in its unphotolyzed (dark) state show an 8 nm difference between Na(+) and K(+) in decyl maltopyranoside micelles, indicating an influence of the cations on the retinylidene photoactive site. In addition, electronic paramagnetic resonance (EPR) spectra of the dark states reveal repositioning of helices F and G when K(+) is replaced with Na(+). Furthermore, the conformational changes assessed by EPR spin-spin dipolar coupling show that the light-induced transmembrane helix movements are very similar to those found in bacteriorhodopsin but are altered by the presence of Na(+), resulting in a new feature, the clockwise rotation of helix F. The results establish the first observation of a cation switch controlling the conformations of a microbial rhodopsin and indicate specific interactions of Na(+) with the half-channels of DeNaR to open an appropriate path for ion translocation.

The sodium/multivitamin transporter: a multipotent system with therapeutic implications

The Na(+)/multivitamin transporter (SMVT) is a member of the solute:sodium symporter family that catalyzes the Na(+)-dependent uptake of the structurally diverse water-soluble vitamins pantothenic acid (vitamin B5) and biotin (vitamin H), α-lipoic acid-a vitamin-like substance with strong antioxidant properties-and iodide. The organic substrates of SMVT play central roles in the cellular metabolism and are, therefore, essential for normal human health and development. For example, biotin deficiency leads to growth retardation, dermatological disorders, and neurological disorders. Animal studies have shown that biotin deficiency during pregnancy is directly correlated to embryonic growth retardation, congenital malformation, and death of the embryo. This chapter focuses on the structural and functional features of the human isoform of SMVT (hSMVT); the discovery of which was greatly facilitated by the cloning and expression of hSMVT in tractable expression systems. Special emphasis will be given to mechanistic implications of the transport process of hSMVT that will inform our understanding of the molecular determinants of hSMVT-mediated transport in dynamic context to alleviate the development and optimization of hSMVT as a multipotent platform for drug delivery.

Sodium-phosphate cotransporter mediates reabsorption of lithium in rat kidney

Lithium, used for the treatment of bipolar disorders, is reabsorbed via sodium-transport system in the proximal tubule. This step causes intra-/inter-individual difference of lithium disposition, and it has not been unclear which transporter contributes. In this study, we examined effect of foscarnet and parathyroid hormone (PTH), inactivators for sodium-phosphate cotransporter, and phlorizin, a typical inhibitor for sodium-glucose cotransporter, on the disposition of lithium in rats. Their intravenous administration stimulated urinary excretion of phosphate or glucose. After the intravenous injection of lithium chloride as a bolus, plasma concentration of lithium decreased time-dependently. The renal clearance of lithium was calculated to be 0.740 ml/min/kg in control rats, and this was 26.7% of creatinine clearance. Foscarnet and PTH significantly increased the renal clearance of lithium and its ratio to creatinine clearance, suggesting that they prevented the reabsorption of lithium. No effect of phlorizin on the renal handling of lithium was recognized. In control rats, the renal clearance of lithium showed a strong correlation with the renal excretion rate of phosphate, compared with creatinine clearance. These findings suggest that sodium-phosphate cotransporter reabsorbs lithium in the rat kidney. Furthermore, its contribution was estimated to be more than 65.9% in the lithium reabsorption. And, this study raised the possibility that therapeutic outcome of lithium is related with the functional expression of sodium-phosphate cotransporter in the kidney.

Fingerprints of hSGLT5 sugar and cation selectivity

Sodium glucose cotransporters (SGLTs) mediate the translocation of carbohydrates across the brush border membrane of different organs such as intestine, kidney, and brain. The human SGLT5 (hSGLT5), in particular, is localized in the kidney were it is responsible for mannose and fructose reabsorption from the glomerular filtrate as confirmed by more recent studies on hSGLT5 knockout mice. Here we characterize the functional properties of hSGLT5 expressed in a stable T-Rex-HEK-293 cell line using biochemical and electrophysiological assays. We confirmed that hSGLT5 is a sodium/mannose transporter that is blocked by phlorizin. Li(+) and H(+) ions were also able to drive mannose transport, and transport was electrogenic. Our results moreover indicate that substrates require a pyranose ring with an axial hydroxyl group (-OH) on carbon 2 (C-2). Compared with Na(+)/glucose cotransport, the level of function of Na(+)/mannose cotransport in rat kidney slices was low.

Functional consequences of NKCC2 splice isoforms: insights from a Xenopus oocyte model

The Na(+)-K(+)-2Cl(-) cotransporter NKCC2 is exclusively expressed in the renal thick ascending limb (TAL), where it exists as three main splice isoforms, NKCC2B, NKCC2A, and NKCC2F, with the latter two predominating. NKCC2A is expressed in both medullary and cortical TAL, but NKCC2F localizes to the medullary TAL. The biochemical characteristics of the isoforms have been extensively studied by ion uptake studies in Xenopus oocytes, but the functional consequences of alternative splicing remain unclear. We developed a charge-difference model of an NKCC2-transfected oocyte. The model closely recapitulated existing data from ion-uptake experiments. This allowed the reconciliation of different apparent Km values reported by various groups, which have hitherto either been attributed to species differences or remained unexplained. Instead, simulations showed that apparent Na(+) and Cl(-) dependencies are influenced by the ambient K(+) or Rb(+) bath concentrations, which differed between experimental protocols. At steady state, under bath conditions similar to the outer medulla, NKCC2F mediated greater Na(+) reabsorption than NKCC2A. Furthermore, Na(+) reabsorption by the NKCC2F-transfected oocyte was more energy efficient, as quantified by J NKCC/J Pump. Both the increased Na(+) reabsorption and the increased efficiency were eroded as osmolarity decreased toward levels observed in the cortical TAL. This supports the hypothesis that the NKCC2F is a medullary specialization of NKCC2 and demonstrates the utility of modeling in analyzing the functional implications of ion uptake data at physiologically relevant steady states.

Mouse SGLT3a generates proton-activated currents but does not transport sugar

Sodium-glucose cotransporters (SGLTs) are secondary active transporters belonging to the SLC5 gene family. SGLT1, a well-characterized member of this family, electrogenically transports glucose and galactose. Human SGLT3 (hSGLT3), despite sharing a high amino acid identity with human SGLT1 (hSGLT1), does not transport sugar, although functions as a sugar sensor. In contrast to humans, two different genes in mice and rats code for two different SGLT3 proteins, SGLT3a and SGLT3b. We previously cloned and characterized mouse SGLT3b (mSGLT3b) and showed that, while it does transport sugar like SGLT1, it likely functions as a physiological sugar sensor like hSGLT3. In this study, we cloned mouse SGLT3a (mSGLT3a) and characterized it by expressing it in Xenopus laevis oocytes and performing electrophysiology and sugar transport assays. mSGLT3a did not transport sugar, and sugars did not induce currents at pH 7.4, though acidic pH induced inward currents that increased in the presence of sugar. Moreover, mutation of residue 457 from glutamate to glutamine resulted in a Na+-dependent transport of sugar that was inhibited by phlorizin. To corroborate our results in oocytes, we expressed and characterized mSGLT3a in mammalian cells and confirmed our findings. In addition, we cloned, expressed, and characterized rat SGLT3a in oocytes and found characteristics similar to mSGLT3a. In summary, acidic pH induces currents in mSGLT3a, and sugar-induced currents are increased at acidic pH, but wild-type SGLT3a does not transport sugar.

Using lithium to probe sequential cation interactions with GAT1

Li(+) interacts with the Na(+)/Cl(-)-dependent GABA transporter, GAT1, under two conditions: in the absence of Na(+) it induces a voltage-dependent leak current; in the presence of Na(+) and GABA, Li(+) stimulates GABA-induced steady-state currents. The amino acids directly involved in the interaction with the Na(+) and Li(+) ions at the so-called "Na2" binding site have been identified, but how Li(+) affects the kinetics of GABA cotransport has not been fully explored. We expressed GAT1 in Xenopus oocytes and applied the two-electrode voltage clamp and (22)Na uptake assays to determine coupling ratios and steady-state and presteady-state kinetics under experimental conditions in which extracellular Na(+) was partially substituted by Li(+). Three novel findings are: 1) Li(+) reduced the coupling ratio between Na(+) and net charge translocated during GABA cotransport; 2) Li(+) increased the apparent Na(+) affinity without changing its voltage dependence; 3) Li(+) altered the voltage dependence of presteady-state relaxations in the absence of GABA. We propose an ordered binding scheme for cotransport in which either a Na(+) or Li(+) ion can bind at the putative first cation binding site (Na2). This is followed by the cooperative binding of the second Na(+) ion at the second cation binding site (Na1) and then binding of GABA. With Li(+) bound to Na2, the second Na(+) ion binds more readily GAT1, and despite a lower apparent GABA affinity, the translocation rate of the fully loaded carrier is not reduced. Numerical simulations using a nonrapid equilibrium model fully recapitulated our experimental findings.

Lithium interactions with Na+-coupled inorganic phosphate cotransporters: insights into the mechanism of sequential cation binding

Type IIa/b Na(+)-coupled inorganic phosphate cotransporters (NaPi-IIa/b) are considered to be exclusively Na(+) dependent. Here we show that Li(+) can substitute for Na(+) as a driving cation. We expressed NaPi-IIa/b in Xenopus laevis oocytes and performed two-electrode voltage-clamp electrophysiology and uptake assays to investigate the effect of external Li(+) on their kinetics. Replacement of 50% external Na(+) with Li(+) reduced the maximum transport rate and the rate-limiting plateau of the P(i)-induced current began at less hyperpolarizing potentials. Simultaneous electrophysiology and (22)Na uptake on single oocytes revealed that Li(+) ions can substitute for at least one of the three Na(+) ions necessary for cotransport. Presteady-state assays indicated that Li(+) ions alone interact with the empty carrier; however, the total charge displaced was 70% of that with Na(+) alone, or when 50% of the Na(+) was replaced by Li(+). If Na(+) and Li(+) were both present, the midpoint potential of the steady-state charge distribution was shifted towards depolarizing potentials. The charge movement in the presence of Li(+) alone reflected the interaction of one Li(+) ion, in contrast to 2 Na(+) ions when only Na was present. We propose an ordered binding scheme for cotransport in which Li(+) competes with Na(+) to occupy the putative first cation interaction site, followed by the cooperative binding of one Na(+) ion, one divalent P(i) anion, and a third Na(+) ion to complete the carrier loading. With Li(+) bound, the kinetics of subsequent partial reactions were significantly altered. Kinetic simulations of this scheme support our experimental data.

Biology of human sodium glucose transporters

There are two classes of glucose transporters involved in glucose homeostasis in the body, the facilitated transporters or uniporters (GLUTs) and the active transporters or symporters (SGLTs). The energy for active glucose transport is provided by the sodium gradient across the cell membrane, the Na(+) glucose cotransport hypothesis first proposed in 1960 by Crane. Since the cloning of SGLT1 in 1987, there have been advances in the genetics, molecular biology, biochemistry, biophysics, and structure of SGLTs. There are 12 members of the human SGLT (SLC5) gene family, including cotransporters for sugars, anions, vitamins, and short-chain fatty acids. Here we give a personal review of these advances. The SGLTs belong to a structural class of membrane proteins from unrelated gene families of antiporters and Na(+) and H(+) symporters. This class shares a common atomic architecture and a common transport mechanism. SGLTs also function as water and urea channels, glucose sensors, and coupled-water and urea transporters. We also discuss the physiology and pathophysiology of SGLTs, e.g., glucose galactose malabsorption and familial renal glycosuria, and briefly report on targeting of SGLTs for new therapies for diabetes.

Determination of the Na(+)/glucose cotransporter (SGLT1) turnover rate using the ion-trap technique

The Na(+)/glucose cotransporter (SGLT1) is a membrane protein that couples the transport of two Na(+) ions and one glucose molecule using the so-called alternating access mechanism. According to this principle, each cotransporter molecule can adopt either of two main conformations: one with the binding sites accessible to the extracellular solution and one with the binding sites facing the intracellular solution. The turnover rate (TOR) is the number of complete cycles that each protein performs per second. Determination of the TOR has important consequences for investigation of the cotransport mechanism, as none of the rate constants involved in mediating transport in a given direction (conformational changes and binding and unbinding reactions) can be slower than the TOR measured under the same conditions. In addition, the TOR can be used to estimate the number of cotransporter molecules involved in generating a given ensemble activity. In this study, we obtain an independent estimation of the TOR for human SGLT1 expressed in Xenopus laevis oocytes applying the ion-trap technique. This approach detects the quantity of ions released in or taken up from the restricted space existing between the oocyte plasma membrane and the tip of a large ion-selective electrode. Taking advantage of the fact that hSGLT1 in the absence of Na(+) can cotransport glucose with protons, we used a pH electrode to determine a TOR of 8.00 ± 1.3 s⁻¹ in the presence of 35 mM α-methyl-glucose at -150 mV (pH 5.5). For the same group of oocytes, a TOR of 13.3 ± 2.4 s⁻¹ was estimated under near-V(max) conditions, i.e., in the presence of 90 mM Na(+) and 5 mM α-methyl-glucose. Under these circumstances, the average cotransport current was -1.08 ± 0.61 μA (n = 14), and this activity was generated by an average of 3.6 ± 0.7 × 10¹¹ cotransporter molecules/oocyte.

Mechanism of melibiose/cation symport of the melibiose permease of Salmonella typhimurium

The MelB permease of Salmonella typhimurium (MelB-ST) catalyzes the coupled symport of melibiose and Na(+), Li(+), or H(+). In right-side-out membrane vesicles, melibiose efflux is inhibited by an inwardly directed gradient of Na(+) or Li(+) and stimulated by equimolar concentrations of internal and external Na(+) or Li(+). Melibiose exchange is faster than efflux in the presence of H(+) or Na(+) and stimulated by an inwardly directed Na(+) gradient. Thus, sugar is released from MelB-ST externally prior to the release of cation in agreement with current models proposed for MelB of Escherichia coli (MelB-EC) and LacY. Although Li(+) stimulates efflux, and an outwardly directed Li(+) gradient increases exchange, it is striking that internal and external Li(+) with no gradient inhibits exchange. Furthermore, Trp → dansyl FRET measurements with a fluorescent sugar (2'-(N-dansyl)aminoalkyl-1-thio-β-D-galactopyranoside) demonstrate that MelB-ST, in the presence of Na(+) or Li(+), exhibits (app)K(d) values of ∼1 mM for melibiose. Na(+) and Li(+) compete for a common binding pocket with activation constants for FRET of ∼1 mM, whereas Rb(+) or Cs(+) exhibits little or no effect. Taken together, the findings indicate that MelB-ST utilizes H(+) in addition to Na(+) and Li(+). FRET studies also show symmetrical emission maximum at ∼500 nm with MelB-ST in the presence of 2'-(N-dansyl)aminoalkyl-1-thio-β-D-galactopyranoside and Na(+), Li(+), or H(+), which implies a relatively homogeneous distribution of conformers of MelB-ST ternary complexes in the membrane.

Surprising substrate versatility in SLC5A6: Na+-coupled I- transport by the human Na+/multivitamin transporter (hSMVT)

Iodide (I(-)) is an essential constituent of the thyroid hormones triiodothyronine and thyroxine, which are required for the development of the central nervous system in the fetus and newborn. I(-) uptake in the thyroid is mediated by the Na(+)/I(-) symporter (NIS). NIS has gained particular medical interest due to its sensitivity to the environmental pollutant perchlorate (ClO(4)(-)) and its implication in radioiodide cancer treatment. Recently, others have shown that I(-) absorption in the intestine is mediated by NIS (Nicola, J. P., Basquin, C., Portulano, C., Reyna-Neyra, A., Paroder, M., and Carrasco, N. (2009) Am. J. Physiol. Cell Physiol. 296, C654-662). However, their data suggest the participation of other systems in the homeostasis of I(-), in particular because in vivo uptake studies revealed a ClO(4)(-)-insensitive transport component. Here, we describe Na(+)-coupled I(-) uptake by the human Na(+)/multivitamin transporter (hSMVT), a related protein isolated from the placenta, where it was suggested to supply the fetus with the water-soluble vitamins biotin and pantothenic acid, and α-lipoic acid. hSMVT-mediated Na(+)/I(-) symport is inhibited by the other three organic hSMVT substrates but not by NIS substrates; notably, hSMVT is insensitive to ClO(4)(-). Because hSMVT is found in the intestine and in many other tissues, we propose that hSMVT may play an important role in the homeostasis of I(-) in the body.

The actual ionic nature of the leak current through the Na+/glucose cotransporter SGLT1

Expression of the Na(+)/glucose cotransporter SGLT1 in Xenopus oocytes is characterized by a phlorizin-sensitive leak current (in the absence of glucose) that was originally called a "Na(+) leak" and represents some 5-10% of the maximal Na(+)/glucose cotransport current. We analyzed the ionic nature of the leak current using a human SGLT1 mutant (C292A) displaying a threefold larger leak current while keeping a reversal potential (V(R)) of approximately -15 mV as observed for wt SGLT1. V(R) showed only a modest negative shift when extracellular Na(+) concentration ([Na(+)](o)) was lowered and it was completely insensitive to changes in extracellular Cl(-). When extracellular pH (pH(o)) was decreased from 7.5 to 6.5 and 5.5, V(R) shifted by +15 and +40 mV, respectively, indicating that protons may be the main charge carrier at low pH(o) but other ions must be involved at pH(o) 7.5. In the presence of 15 mM [Na(+)](o) (pH(o) = 7.5), addition of 75 mM of either Na(+), Li(+), Cs(+), or K(+) generated similar increases in the leak current amplitude. This observation, which was confirmed with wt SGLT1, indicates a separate pathway for the leak current with respect to the cotransport current. This means that, contrary to previous beliefs, the leak current cannot be accounted for by the translocation of the Na-loaded and glucose-free cotransporter. Using chemical modification and different SGLT1 mutants, a relationship was found between the cationic leak current and the passive water permeability suggesting that water and cations may share a common pathway through the cotransporter.

Structure and function of Na(+)-symporters with inverted repeats

Symporters are membrane proteins that couple energy stored in electrochemical potential gradients to drive the cotransport of molecules and ions into cells. Traditionally, proteins are classified into gene families based on sequence homology and functional properties, for example the sodium glucose (SLC5 or Sodium Solute Symporter Family, SSS or SSF) and GABA (SLC6 or Neurotransmitter Sodium Symporter Family, NSS or SNF) symporter families [1-4]. Recently, it has been established that four Na(+)-symporter proteins with unrelated sequences have a common structural core containing an inverted repeat of 5 transmembrane (TM) helices [5(**)-8(**)]. Analysis of these four structures reveals that they reside in different conformations along the transport cycle providing atomic insight into the mechanism of sodium solute cotransport.

Red fluorescent protein pH biosensor to detect concentrative nucleoside transport

Human concentrative nucleoside transporter, hCNT3, mediates Na+/nucleoside and H+/nucleoside co-transport. We describe a new approach to monitor H+/uridine co-transport in cultured mammalian cells, using a pH-sensitive monomeric red fluorescent protein variant, mNectarine, whose development and characterization are also reported here. A chimeric protein, mNectarine fused to the N terminus of hCNT3 (mNect.hCNT3), enabled measurement of pH at the intracellular surface of hCNT3. mNectarine fluorescence was monitored in HEK293 cells expressing mNect.hCNT3 or mNect.hCNT3-F563C, an inactive hCNT3 mutant. Free cytosolic mNect, mNect.hCNT3, and the traditional pH-sensitive dye, BCECF, reported cytosolic pH similarly in pH-clamped HEK293 cells. Cells were incubated at the permissive pH for H(+)-coupled nucleoside transport, pH 5.5, under both Na(+)-free and Na(+)-containing conditions. In mNect.hCNT3-expressing cells (but not under negative control conditions) the rate of acidification increased in media containing 0.5 mm uridine, providing the first direct evidence for H(+)-coupled uridine transport. At pH 5.5, there was no significant difference in uridine transport rates (coupled H+ flux) in the presence or absence of Na+ (1.09 +/- 0.11 or 1.18 +/- 0.32 mm min(-1), respectively). This suggests that in acidic Na(+)-containing conditions, 1 Na+ and 1 H+ are transported per uridine molecule, while in acidic Na(+)-free conditions, 1 H+ alone is transported/uridine. In acid environments, including renal proximal tubule, H+/nucleoside co-transport may drive nucleoside accumulation by hCNT3. Fusion of mNect to hCNT3 provided a simple, self-referencing, and effective way to monitor nucleoside transport, suggesting an approach that may have applications in assays of transport activity of other H(+)-coupled transport proteins.

Elucidating conformational changes in the gamma-aminobutyric acid transporter-1

The GABA transporter-1 (GAT-1) has three current-generating modes: GABA-coupled current, Li+-induced leak current, and Na+-dependent transient currents. We earlier hypothesized that Li+ is able to substitute for the first Na+ in the transport cycle and thereby induce a distinct conformation in GAT-1 and that the onset of the Li+-induced leak current at membrane potentials more negative than -50 mV was due to a voltage-dependent conformational change of the Li+-bound transporter. In this study, we set out to verify this hypothesis and seek insight into the structural dynamics underlying the leak current, as well as the sodium-dependent transient currents, by applying voltage clamp fluorometry to tetramethylrhodamine 6-maleimide-labeled GAT-1 expressed in Xenopus laevis oocytes. MTSET accessibility studies demonstrated the presence of two distinct conformations of GAT-1 in the presence of Na+ or Li+. The voltage-dependent fluorescence intensity changes obtained in Li+ buffer correlated with the Li+-induced leak currents, i.e. both were highly voltage-dependent and only present at hyperpolarized potentials (<-50 mV). The transient currents correlated directly with the voltage-dependent fluorescence data obtained in sodium buffer and the associated conformational changes were distinct from those associated with the Li+-induced leak current. The inhibitor potency of SKF89976A of the Li+- versus Na+-bound transporter confirmed the cationic dependence of the conformational occupancy. Our observations suggest that the microdomain situated at the external end of transmembrane I is involved in different conformational changes taking place either during the binding and release of sodium or during the initiation of the Li+-induced leak current.

Measuring ion transport activities in Xenopus oocytes using the ion-trap technique

The ion-trap technique is an experimental approach allowing measurement of changes in ionic concentrations within a restricted space (the trap) comprised of a large-diameter ion-selective electrode apposed to a voltage-clamped Xenopus laevis oocyte. The technique is demonstrated with oocytes expressing the Na(+)/glucose cotransporter (SGLT1) using Na(+)- and H(+)-selective electrodes and with the electroneutral H(+)/monocarboxylate transporter (MCT1). In SGLT1-expressing oocytes, bath substrate diffused into the trap within 20 s, stimulating Na(+)/glucose influx, which generated a measurable decrease in the trap Na(+) concentration ([Na(+)](T)) by 0.080 +/- 0.009 mM. Membrane hyperpolarization produced a further decrease in [Na(+)](T), which was proportional to the increased cotransport current. In a Na(+)-free, weakly buffered solution (pH 5.5), H(+) drives glucose transport through SGLT1, and this was monitored with a H(+)-selective electrode. Proton movements can also be clearly detected on adding lactate to an oocyte expressing MCT1 (pH 6.5). For SGLT1, time-dependent changes in [Na(+)](T) or [H(+)](T) were also detected during a membrane potential pulse (150 ms) in the presence of substrate. In the absence of substrate, hyperpolarization triggered rapid reorientation of SGLT1 cation binding sites, accompanied by cation capture from the trap. The resulting change in [Na(+)](T) or [H(+)](T) is proportional to the pre-steady-state charge movement. The ion-trap technique can thus be used to measure steady-state and pre-steady-state transport activities and provides new opportunities for studying electrogenic and electroneutral ion transport mechanisms.

Functional identification of a novel transport system for endogenous and synthetic opioid peptides in the rabbit conjunctival epithelial cell line CJVE

PURPOSE

To investigate whether conjunctival epithelial cells express transport processes for opioid peptides.

METHODS

We monitored the uptake of [(3)H]deltorphin II and [(3)H]DADLE, two hydrolysis-resistant synthetic opioid peptides, in the rabbit conjunctival epithelial cell line CJVE and elucidated the characteristics of the uptake process.

RESULTS

CJVE cells express robust uptake activity for deltorphin II and DADLE. Both opioid peptides compete with each other for transport. Several endogenous and synthetic opioid peptides, but not non-peptide opioid antagonists, are recognized by the transport process. Though various peptides inhibit the uptake of deltorphin II and DADLE in a similar manner, the uptake of deltorphin II is partly Na(+)-dependent whereas that of DADLE mostly Na(+)-independent. The transport process shows high affinity for many endogenous/synthetic opioid peptides. Functional features reveal that this transport process may be distinct from the opioid peptide transport system described in the retinal pigment epithelial cell line ARPE-19 and also from the organic anion transporting polypeptides, which are known to transport opioid peptides.

CONCLUSIONS

CJVE cells express a novel, hitherto unknown transport process for endogenous/synthetic opioid peptides. This new transport process may offer an effective delivery route for opioid peptide drugs to the posterior segment of the eye.

How drugs interact with transporters: SGLT1 as a model

Drugs are transported by cotransporters with widely different turnover rates. We have examined the underlying mechanism using, as a model system, glucose and indican (indoxyl-beta-D-glucopyranoside) transport by human Na+/glucose cotransporter (hSGLT1). Indican is transported by hSGLT1 at 10% of the rate for glucose but with a fivefold higher apparent affinity. We expressed wild-type hSGLT1 and mutant G507C in Xenopus oocytes and used electrical and optical methods to measure the kinetics of glucose (using nonmetabolized glucose analogue alpha-methyl-D-glucopyranoside, alphaMDG) and indican transport, alone and together. Indican behaved as a competitive inhibitor of alphaMDG transport. To examine protein conformations, we recorded SGLT1 capacitive currents (charge movements) and fluorescence changes in response to step jumps in membrane voltage, in the presence and absence of indican and/or alphaMDG. In the absence of sugar, voltage jumps elicited capacitive SGLT currents that decayed to steady state with time constants (tau) of 3-20 ms. These transient currents were abolished in saturating alphaMDG but only slightly reduced (10%) in saturating indican. SGLT1 G507C rhodamine fluorescence intensity increased with depolarizing and decreased with hyperpolarizing voltages. Maximal fluorescence increased approximately 150% in saturating indican but decreased approximately 50% in saturating alphaMDG. Modeling indicated that the rate-limiting step for indican transport is sugar translocation, whereas for alphaMDG it is dissociation of Na+ from the internal binding sites. The inhibitory effects of indican on alphaMDG transport are due to its higher affinity and a 100-fold lower translocation rate. Our results indicate that competition between substrates and drugs should be taken into consideration when targeting transporters as drug delivery systems.

Phosphate transporters: a tale of two solute carrier families

Phosphate is an essential component of life and must be actively transported into cells against its electrochemical gradient. In vertebrates, two unrelated families of Na+ -dependent P(i) transporters carry out this task. Remarkably, the two families transport different P(i) species: whereas type II Na+/P(i) cotransporters (SCL34) prefer divalent HPO(4)(2-), type III Na(+)/P(i) cotransporters (SLC20) transport monovalent H2PO(4)(-). The SCL34 family comprises both electrogenic and electroneutral members that are expressed in various epithelia and other polarized cells. Through regulated activity in apical membranes of the gut and kidney, they maintain body P(i) homeostasis, and in salivary and mammary glands, liver, and testes they play a role in modulating the P(i) content of luminal fluids. The two SLC20 family members PiT-1 and PiT-2 are electrogenic and ubiquitously expressed and may serve a housekeeping role for cell P(i) homeostasis; however, also more specific roles are emerging for these transporters in, for example, bone mineralization. In this review, we focus on recent advances in the characterization of the transport kinetics, structure-function relationships, and physiological implications of having two distinct Na+/P(i) cotransporter families.

Substrate specificity of sugar transport by rabbit SGLT1: single-molecule atomic force microscopy versus transport studies

In the apical membrane of epithelial cells from the small intestine and the kidney, the high-affinity Na+/d-glucose cotransporter SGLT1 plays a crucial role in selective sugar absorption and reabsorption. How sugars are selected at the molecular level is, however, poorly understood. Here atomic force microscopy (AFM) was employed to investigate the substrate specificity of rbSGLT1 on the single-molecule level, while competitive-uptake assays with isotope-labeled sugars were performed in the study of the stereospecificity of the overall transport. rbSGLT1-transfected Chinese hamster ovary (CHO) cells were used for both approaches. Evidence of binding of d-glucose to the extracellular surface of rbSGLT1 could be obtained using AFM tips carrying 1-thio-d-glucose coupled at the C1 position to a PEG linker via a vinylsulfon group. Competition experiments with monosaccharides in solution revealed the following selectivity ranking of binding: 2-deoxy-d-glucose >or= 6-deoxy-d-glucose > d-glucose > d-galactose >or= alpha-methyl glucoside; 3-deoxy-d-glucose, d-xylose, and l-glucose did not measurably affect binding. These results were different from those of competitive alpha-methyl glucoside transport assays, where the ranking of inhibition was as follows: d-glucose > d-galactose > 6-deoxy-d-glucose; no uptake inhibition by d-xylose, 3-deoxy-d-glucose, 2-deoxy-d-glucose, or l-glucose was observed. Taken together, these results suggest that the substrate specificity of SGLT1 is determined by different recognition sites: one possibly located at the surface of the transporter and others located close to or within the translocation pathway.

Specific Mutations in Transmembrane Helix 8 of Human Concentrative Na+/Nucleoside Cotransporter hCNT1 Affect Permeant Selectivity and Cation Coupling

The Na+/nucleoside cotransporters hCNT1 (650 residues) and hCNT2 (658 residues) are 72% identical in amino acid sequence and contain 13 putative transmembrane helices (TMs). Both transport uridine and adenosine but are otherwise selective for pyrimidine (system cit) and purine (system cif) nucleosides, respectively. Previously, we used site-directed mutagenesis and functional expression in Xenopus oocytes to identify two pairs of adjacent residues in TMs 7 and 8 of hCNT1 (Ser319-Gln320 and Ser353-Leu354) that, when converted to the corresponding residues in hCNT2 (Gly-Met and Thr-Val, respectively), changed the permeant selectivity of the transporter from cit to cif. We now report an investigation of the effects of corresponding mutations in TM 8 alone and demonstrate unique S353T- and L354V-induced changes in nucleoside specificity and cation coupling, respectively. hCNT1 mutation S353T produced a profound decrease in cytidine transport efficiency (Vmax/Km ratio) and, in combination with L354V (S353T/L354V), resulted in a novel uridine-preferring transport phenotype. In addition, the L354V mutation markedly increased the apparent affinity of hCNT1 for Na+ and Li+. Both hCNT1 TM 8 residues exhibited uridine-protectable inhibition by p-chloromercuribenzene sulfonate when converted to Cys, suggesting that they occupy positions within or closely adjacent to a common cation/nucleoside translocation pore.

Mechanistic Insights and Functional Determinants of the Transport Cycle of the Ascorbic Acid Transporter SVCT2

We characterized the human Na(+)-ascorbic acid transporter SVCT2 and developed a basic model for the transport cycle that challenges the current view that it functions as a Na(+)-dependent transporter. The properties of SVCT2 are modulated by Ca(2+)/Mg(2+) and a reciprocal functional interaction between Na(+) and ascorbic acid that defines the substrate binding order and the transport stoichiometry. Na(+) increased the ascorbic acid transport rate in a cooperative manner, decreasing the transport K(m) without affecting the V(max), thus converting a low affinity form of the transporter into a high affinity transporter. Inversely, ascorbic acid affected in a bimodal and concentration-dependent manner the Na(+) cooperativity, with absence of cooperativity at low and high ascorbic acid concentrations. Our data are consistent with a transport cycle characterized by a Na(+):ascorbic acid stoichiometry of 2:1 and a substrate binding order of the type Na(+):ascorbic acid:Na(+). However, SVCT2 is not electrogenic. SVCT2 showed an absolute requirement for Ca(2+)/Mg(2+) for function, with both cations switching the transporter from an inactive into an active conformation by increasing the transport V(max) without affecting the transport K(m) or the Na(+) cooperativity. Our data indicate that SVCT2 may switch between a number of states with characteristic properties, including an inactive conformation in the absence of Ca(2+)/Mg(2+). At least three active states can be envisioned, including a low affinity conformation at Na(+) concentrations below 20 mM and two high affinity conformations at elevated Na(+) concentrations whose Na(+) cooperativity is modulated by ascorbic acid. Thus, SVCT2 is a Ca(2+)/Mg(2+)-dependent transporter.

Ligands on the string: single-molecule AFM studies on the interaction of antibodies and substrates with the Na+-glucose co-transporter SGLT1 in living cells

Atomic force microscopy (AFM) was used to probe topology, conformational changes and initial substratecarrier interactions of Na+-glucose co-transporter (SGLT1) in living cells on a single-molecule level. By scanning SGLT1-transfected Chinese hamster ovary (CHO) cells with AFM tips carrying an epitope-specific antibody directed against the extramembranous C-terminal loop 13, significant recognition events could be detected. Specificity was confirmed by the absence of events in nontransfected CHO cells and by the use of free antigen and free antibody superfusion. Thus, contrary to computer predictions on SGLT1 topology, loop 13 seems to be part of the extracellular surface of the transporter. Binding probability of the antibody decreased upon addition of phlorizin, a specific inhibitor of SGLT1, suggesting a considerable conformational change of loop 13 when the inhibitor occludes the sugar translocation pathway. Using an AFM tip carrying 1-thio-D-glucose, direct evidence could be obtained that in the presence of Na+ a sugarbinding site appears on the transporter surface. The binding site accepts the sugar residue of the glucoside phlorizin, free D-glucose, and D-galactose, but not free Lglucose and probably represents the first of several selectivity filters of the transporter. This work demonstrates the potential of AFM to study the presence and dynamics of plasma membrane transporters in intact cells on the single molecule level.

Effets secondaires de faibles concentrations de lithium sur les fonctions rénales, thyroïdiennes et sexuelles chez des rats matures mâles et femelles

The present study, carried out in rats, is a contribution to explore physiological mechanisms underlying lithium toxicity. Male and female mature rats were divided into three groups and fed on commercial pellets: group (C) was control, group (Li1) was given 2000 mg lithium carbonate/kg of food, and group (Li2) was given 4000 mg lithium carbonate/kg of food. If we take into account the BW of the rats and the quantity of food they eat every day, we can estimate that the quantities of lithium carbonate ingested per day and kilogram of BW are, respectively, for the groups Li1 and Li2, of 212 mg (5,738 mmol Li) and 323 mg (8,742 mmol Li) for the males, and about 190 mg (5,142 mmol Li) and 289 mg (7,822 mmol Li) for the females. After 7, 14, 21 and 28 days, serum concentrations of lithium, creatinine, free triiodothyronine (FT3) and thyroxine (FT4), testosterone and estradiol were measured. Attention was also paid to growth rate and a histological examination of testes or vaginal mucosa was carried out. In treated rats, a dose-dependent loss of appetite and a decrease in growth rate were observed together with polydipsia, polyuria, and diarrhoea. Lithium serum concentrations were found to increase from 0.44 mM (day 7) to 1.34 mM (day 28) in Li1 rats and from 0.66 to 1.45 mM (day 14) in Li2 rats. Treatment was stopped at day 14 in Li2 rats because of a high mortality. The significant increase of creatinine that appeared, respectively, at day 7 and 14 in Li2 and Li1 rats shows that serum lithium concentrations ranging from 0.62 to 0.75 mM were able to induce renal insufficiency, secondarily leading to a time-dependent rise in lithium serum concentrations. A significant decrease of serum thyroxine (FT4) and triiodothyronine (FT3) levels was observed for lithium concentrations ranging from: 0.66 to 0.75 mmol l(-1) (Li2 rats) to 1.27 mmol l(-1) (Li1 rats). This effect was more pronounced for FT3, suggesting a defect of FT4/FT3 conversion. Under lithium treatment, the testosterone level decreased and spermatogenesis was stopped. By contrast, in treated female rats, estradiol level was found to be increased in a dose-dependent manner and animals were blocked in the diestrus phase at day 28. These results show that lithium can rapidly induce toxic effects in the rat at concentrations used for the treatment of bipolar disorders in human.

Kinetics of the reverse mode of the Na+/glucose cotransporter

This study investigates the reverse mode of the Na(+)/glucose cotransporter (SGLT1). In giant excised inside-out membrane patches from Xenopus laevis oocytes expressing rabbit SGLT1, application of alpha-methyl-D: -glucopyranoside (alphaMDG) to the cytoplasmic solution induced an outward current from cytosolic to external membrane surface. The outward current was Na(+)- and sugar-dependent, and was blocked by phlorizin, a specific inhibitor of SGLT1. The current-voltage relationship saturated at positive membrane voltages (30-50 mV), and approached zero at -150 mV. The half-maximal concentration for alphaMDG-evoked outward current (K(0.5) (alphaMDG)) was 35 mM (at 0 mV). In comparison, K(0.5) (alphaMDG) for forward sugar transport was 0.15 mM (at 0 mV). K(0.5) (Na) was similar for forward and reverse transport ( approximately 35 mM at 0 mV). Specificity of SGLT1 for reverse transport was: alphaMDG (1.0) > D: -galactose (0.84) > 3-O-methyl-glucose (0.55) > D: -glucose (0.38), whereas for forward transport, specificity was: alphaMDG approximately D: -glucose approximately D: -galactose > 3-O-methyl-glucose. Thus there is an asymmetry in sugar kinetics and specificity between forward and reverse modes. Computer simulations showed that a 6-state kinetic model for SGLT1 can account for Na(+)/sugar cotransport and its voltage dependence in both the forward and reverse modes at saturating sodium concentrations. Our data indicate that under physiological conditions, the transporter is poised to accumulate sugar efficiently in the enterocyte.

The Broadly Selective Human Na+/Nucleoside Cotransporter(hCNT3) Exhibits Novel Cation-coupled Nucleoside TransportCharacteristics

The concentrative nucleoside transporter (CNT) protein family in humans is represented by three members, hCNT1, hCNT2, and hCNT3. hCNT3, a Na+/nucleoside symporter, transports a broad range of physiological purine and pyrimidine nucleosides as well as anticancer and antiviral nucleoside drugs, and belongs to a different CNT subfamily than hCNT1/2. H+-dependent Escherichia coli NupC and Candida albicans CaCNT are also CNT family members. The present study utilized heterologous expression in Xenopus oocytes to investigate the specificity, mechanism, energetics, and structural basis of hCNT3 cation coupling. hCNT3 exhibited uniquely broad cation interactions with Na+, H+, and Li+ not shared by Na+-coupled hCNT1/2 or H+-coupled NupC/CaCNT. Na+ and H+ activated hCNT3 through mechanisms to increase nucleoside apparent binding affinity. Direct and indirect methods demonstrated cation/nucleoside coupling stoichiometries of 2:1 in the presence of Na+ and both Na+ plus H+, but only 1:1 in the presence of H+ alone, suggesting that hCNT3 possesses two Na+-binding sites, only one of which is shared by H+. The H+-coupled hCNT3 did not transport guanosine or 3'-azido-3'-deoxythymidine and 2',3'-dideoxycytidine, demonstrating that Na+- and H+-bound versions of hCNT3 have significantly different conformations of the nucleoside binding pocket and/or translocation channel. Chimeric studies between hCNT1 and hCNT3 located hCNT3-specific cation interactions to the C-terminal half of hCNT3, setting the stage for site-directed mutagenesis experiments to identify the residues involved.

Coupled Sodium/Glucose Cotransport by SGLT1 Requires a Negative Charge at Position 454

Na(+)/glucose cotransport by SGLT1 is a tightly coupled process that is driven by the Na(+) electrochemical gradient across the plasma membrane. We have previously proposed that SGLT1 contains separate Na(+)- and glucose-binding domains, that A166 (in the Na(+) domain) is close to D454 (in the sugar domain), and that interactions between these residues influence sugar specificity and transport. We have now expressed the mutant D454C in Xenopus laevis oocytes and examined the role of charge on residue 454 by replacing the Asp with Cys or His, and by chemical mutation of D454C with alkylating reagents of different charge (MTSES(-), MTSET(+), MMTS(0), MTSHE(0), and iodoacetate(-)). Functional properties were examined by measuring sugar transport and cotransporter currents. In addition, D454C was labeled with fluorescent dyes and the fluorescence of the labeled transporter was recorded as a function of voltage and ligand concentration. The data shows that (1) aspartate 454 is critically important for the normal trafficking of the protein to the plasma membrane; (2) there were marked changes in the functional properties of D454C, i.e., a reduction in turnover number and a loss of voltage sensitivity, although there were no alterations in sugar selectivity or sugar and Na(+) affinity; (3) a negative charge on residue 454 increased Na(+) and sugar transport with a normal stoichiometry of 2 Na(+):1 sugar. A positive charge on residue 454, in contrast, uncoupled Na(+) and sugar transport, indicating the importance of the negative charge in the coordination of the cotransport mechanism.

C-Terminus Loop 13 of Na+ Glucose Cotransporter SGLT1 Contains a Binding Site for Alkyl Glucosides

Recently, we identified the extramembranous C-terminus loop 13 of SGLT1 as a binding site for the aromatic glucoside phlorizin, which competitively inhibits sodium D-glucose cotransport. Alkyl glucosides are also competitive inhibitors of the transport. Therefore, in this study, we searched for potential binding sites for alkyl glucosides in loop 13. To this end, we synthesized a photoaffinity label (2'-Azi-n-octyl)-beta-D-glucoside and analyzed the region of attachment using MALDI mass spectrometry, producing wild-type recombinant truncated loop 13. Furthermore, we prepared four single-Trp mutants of the loop and determined their fluorescence, its change in the presence of alkyl glucosides, and their accessibility to acrylamide. Photolabeling of truncated loop 13 with (2'-Azi-n-octyl)-beta-D-glucoside revealed an attachment of the C2 group of the alkyl chain to Gly-Phe-Phe-Arg (amino acid residues 598-601). In the presence of n-hexyl-beta-D-glucoside, all mutants (R601W, D611W, E621W, and L630W) exhibited a significant decrease in Trp fluorescence with an apparent binding affinity of 8-14 microM. Only L630W exhibited a significant blue shift, and only in R601W was a change in acrylamide quenching (protection) observed. No quenching or protection was found for D-glucose; however, 1-hexanol produced the same results as n-hexyl-beta-D-glucoside. The interaction shows stereoselectivity for n-hexyl-beta-D-glucoside binding; the beta-configuration of the sugar moiety at C1, the cis conformation of the unsaturated alkenyl side chain in the C3-C4 bond, and the alkyl chain length of six to eight carbon atoms lead to an optimum interaction. A schematic two-dimensional model was derived in which C2 interacts with the region around residue 601, C3 and C4 interact with the region between residues 614 and 619, and C6-C8 interact with the region between residues 621 and 630. The data demonstrate that loop 13 provides binding sites for alkyl glucosides as well as for phlorizin; thus, loop 13 of SGLT1 seems to be a major binding domain for the aglucone residues of competitive D-glucose transport inhibitors.

The sodium/glucose cotransport family SLC5

The sodium/glucose cotransporter family (SLCA5) has 220 or more members in animal and bacterial cells. There are 11 human genes expressed in tissues ranging from epithelia to the central nervous system. The functions of nine have been revealed by studies using heterologous expression systems: six are tightly coupled plasma membrane Na(+)/substrate cotransporters for solutes such as glucose, myo-inositol and iodide; one is a Na(+)/Cl(-)/choline cotransporter; one is an anion transporter; and another is a glucose-activated ion channel. The exon organization of eight genes is similar in that each comprises 14-15 exons. The choline transporter (CHT) is encoded in eight exons and the Na(+)-dependent myo-inositol transporter (SMIT) in one exon. Mutations in three genes produce genetic diseases (glucose-galactose malabsorption, renal glycosuria and hypothyroidism). Members of this family are multifunctional membrane proteins in that they also behave as uniporters, urea and water channels, and urea and water cotransporters. Consequently it is a challenge to determine the role(s) of these genes in human physiology and pathology.

Phlorizin recognition in a C-terminal fragment of SGLT1 studied by tryptophan scanning and affinity labeling

SGLT1 as a sodium/glucose cotransporter is strongly inhibited by phlorizin, a phloretin 2'-glucoside that has strong interactions with the C-terminal loop 13. We have examined phlorizin recognition by the protein by site-directed single Trp scanning mutagenesis experiments. Six mutants (Q581W, E591W, R601W, D611W, E621W, and L630W) of truncated loop 13 (amino acids 564-638) were expressed in Escherichia coli and purified to homogeneity. Changes in Trp quenching and positions of the emission maxima were determined after addition of phlorizin. D611W displayed the largest quenching of 80%, followed by R601W (67%). It also exhibited the maximum red shift in Trp fluorescence ( approximately 14 nm), indicating an exposure of this region to a more hydrophilic environment. Titration experiments performed for each mutant showed a similar affinity for all mutants, except for D611W, which exhibited a significantly lower affinity (Kd approximately 54 microm). Also the maximum change in the collisional quenching constant by acrylamide was noted for D611W (KSV = 11 m-1 in the absence of phlorizin and 55 m-1 in its presence). Similar results were obtained with phloretin. CD measurements and computer modeling revealed that D611W is positioned in a random coil situated between two alpha-helical segments. By combining gel electrophoresis, enzymatic fragmentation, and matrix-assisted laser desorption ionization mass spectrometry, we also analyzed truncated loop 13 photolabeled with 3-azidophlorizin. The attachment site of the ortho-position of aromatic ring B of phlorizin was localized to Arg-602. Taken together, these data indicate that phlorizin binding elicits changes in conformation leading to a less ordered state of loop 13. Modeling suggests an interaction of the 4- and 6-OH groups of aromatic ring A of phlorizin with the region between amino acids 606 and 611 and an interaction of ring B at or around amino acid 602. Phloretin seems to interact with the same region of the protein.

Binding of phlorizin to the isolated C-terminal extramembranous loop of the Na+/glucose cotransporter assessed by intrinsic tryptophan fluorescence

Phlorizin, a phloretin 2'-glucoside, is a potent inhibitor of the Na(+)/glucose cotransporter (SGLT1). On the basis of transport studies in intact cells, a binding site for phlorizin was suggested in the region between amino acids 604-610 of the C-terminal loop 13. To further investigate phlorizin binding titration experiments of the intrinsic Trp fluorescence of isolated wild-type loop 13 and two mutated loops (Y604K and G609K) were carried out. Phlorizin (135 microM) produced approximately 40% quenching of the fluorescence of wild-type loop 13; quenching could also be observed with the two mutated loops. The apparent K(d) was lowest for the wild-type loop 13 (K(d) approximately 23 microM), followed by mutant G609K (57 microM) and mutant Y604K (70 microM). Binding of phlorizin was further confirmed by a decrease of the accessibility of loop 13 to the collisional quencher acrylamide. The interaction involves the aromatic moiety of the aglucone since phloretin (the aglucone of phlorizin) showed almost the same effects as phlorizin, while d-glucose did not. MALDI-TOF experiments revealed that loop 13 contained a disulfide bond between Cys 560 and Cys 608 that is very important for phlorizin-dependent fluorescence quenching. These studies provide direct evidence that loop 13 is a site (important amino acids including 604-609) for the molecular interaction between SGLT1 and phlorizin. They confirm that the aglucone part of the glucoside is responsible for this interaction.

The sodium/glucose cotransport family SLC5

The sodium/glucose cotransporter family (SLCA5) has 220 or more members in animal and bacterial cells. There are 11 human genes expressed in tissues ranging from epithelia to the central nervous system. The functions of nine have been revealed by studies using heterologous expression systems: six are tightly coupled plasma membrane Na(+)/substrate cotransporters for solutes such as glucose, myo-inositol and iodide; one is a Na(+)/Cl(-)/choline cotransporter; one is an anion transporter; and another is a glucose-activated ion channel. The exon organization of eight genes is similar in that each comprises 14-15 exons. The choline transporter (CHT) is encoded in eight exons and the Na(+)-dependent myo-inositol transporter (SMIT) in one exon. Mutations in three genes produce genetic diseases (glucose-galactose malabsorption, renal glycosuria and hypothyroidism). Members of this family are multifunctional membrane proteins in that they also behave as uniporters, urea and water channels, and urea and water cotransporters. Consequently it is a challenge to determine the role(s) of these genes in human physiology and pathology.

"Causes" in chemical explanations

Chemists talk about causes and make frequent use of the verb "to cause." They use the verb "to cause" for its generality in identifying causal chains, whether all the details of those chains are established or some remain unclear. Most of these causal chains bottom out in attractions and repulsions. Attractions and repulsions are causal relations that are chemically basic; chemistry has no further story to tell about why attraction and repulsion are causal. If there is some further story to tell about the causal status of attraction and repulsion, it is left to physics to tell it, but it is not obvious that the absence of such a story would prevent chemists from using attractions and repulsions in their causal explanations.

The sodium/substrate symporter family: structural and functional features

Members of the sodium/substrate symporter family (SSSF, TC 2.A.21) catalyze the uptake of a wide variety of solutes including sugars, proline, pantothenate, and iodide into cells of pro- and eukaryotic origin. Extensive analyses of the topology of different SSSF proteins suggest an arrangement of 13 transmembrane domains as a common topological motif. Regions involved in sodium and/or substrate binding were identified. Furthermore, protein chemical and spectroscopic studies reveal ligand-induced structural alterations which are consistent with close interactions between the sites of sodium and substrate binding, thereby supporting an ordered binding mechanism for transport.

Evidence for intraprotein charge transfer during the transport activity of the melibiose permease from Escherichia coli

Electrogenic activity associated with the activity of the melibiose permease (MelB) of Escherichia coli was investigated by using proteoliposomes containing purified MelB adsorbed onto a solid-supported membrane. Transient currents were selectively recorded by applying concentration jumps of Na+ ions (or Li+) and/or of different sugar substrates of MelB (melibiose, thio-methyl galactoside, raffinose) using a fast-flow solution exchange system. Characteristically, the transient current response was fast, including a single decay exponential component (tau approximately 15 ms) on applying a Na+ (or Li+) concentration jump in the absence of sugar. On imposing a Na+ (or Li+) jump on proteoliposomes preincubated with the sugar, a sugar jump in a preparation preincubated with the cation, or a simultaneous jump of the cation and sugar substrates, the electrical transients were biphasic and comprised both the fast and an additional slow (tau approximately 350 ms) decay components. Finally, selective inactivation of the cosubstrate translocation step by acylation of MelB cysteins with N-ethyl maleimide suppressed the slow response components and had no effect on the fast transient one. We suggest that the fast transient response reflects charge transfer within MelB during cosubstrate binding while the slow component is associated with charge transfer across the proteoliposome membrane. From the time course of the transient currents, we estimate a rate constant for Na+ binding in the absence and presence of melibiose of k > 50 s(-1) and one for melibiose binding in the absence of Na+ of k approximately 10 s(-1).

Evidence for the involvement of Ala 166 in coupling Na(+) to sugar transport through the human Na(+)/glucose cotransporter

We mutated residue 166, located in the putative Na(+) transport pathway between transmembrane segments 4 and 5 of human Na(+)/glucose cotransporter (hSGLT1), from alanine to cysteine (A166C). A166C was expressed in Xenopus laevis oocytes, and electrophysiological methods were used to assay function. The affinity for Na(+) was unchanged compared to that of hSGLT1, whereas the sugar affinity was reduced and sugar specificity was altered. There was a reduction in the turnover rate of the transporter, and in contrast to that of hSGLT1, the turnover rate depended on the sugar molecule. Exposure of A166C to MTSEA and MTSET, but not MTSES, abolished sugar transport. Accessibility of A166C to alkylating reagents was independent of protein conformation, indicating that the residue is always accessible from the extracellular surface. Sugar and phlorizin did not protect the residue from being alkylated, suggesting that residue 166 is not located in the sugar pathway. MTSEA, MTSET, and MTSES all changed the pre-steady-state kinetics of A166C, independent of pH, and sugars altered these kinetics. The inability of MTSEA-labeled A166C to transport sugar was reversed (with no major change in Na(+) and sugar affinity) if the positive charge on MTSEA was neutralized by increasing the external pH to 9.0. These studies suggest that the residue at position 166 is involved in the interaction between the Na(+) and sugar transport pathways.

Neutralization of a conserved amino acid residue in the human Na+/glucose transporter (hSGLT1) generates a glucose-gated H+ channel

The role of conserved Asp204 in the human high affinity Na+/glucose cotransporter (hSGLT1) was investigated by site-directed mutagenesis combined with functional assays exploiting the Xenopus oocyte expression system. Substitution of H+ for Na+ reduces the apparent affinity of hSGLT1 for glucose from 0.3 to 6 mm. The apparent affinity for H+ (7 microm) is about three orders of magnitude higher than for Na+ (6 mm). Cation/glucose cotransport exhibits a coupling ratio of 2 Na+ (or 2 H+):1. Pre-steady-state kinetics indicate that similar Na+ - or H+ -induced conformational changes are the basis for coupled transport. Replacing Asp204 with Glu increases the apparent affinity for H+ by >20-fold with little impact on the apparent Na+ affinity. This implies that the length of the carboxylate side chain is critical for cation selectivity. Neutralization of Asp204 (Asp --> Asn or Cys) reveals glucose-evoked H(+) currents that were one order of magnitude greater than Na(+) currents. These phlorizin-sensitive H+ currents reverse and are enhanced by internal acidification of oocytes. Together with a H(+) to sugar stoichiometry as high as 145:1, these results favor a glucose-gated H+ channel activity of the mutant. Our observations support the idea that cotransporters and channels share common features.

Renal Na(+)-glucose cotransporters

In humans, the kidneys filter approximately 180 g of D-glucose from plasma each day, and this is normally reabsorbed in the proximal tubules. Although the mechanism of reabsorption is well understood, Na(+)-glucose cotransport across the brush-border membrane and facilitated diffusion across the basolateral membrane, questions remain about the identity of the genes responsible for cotransport across the brush border. Genetic studies suggest that two different genes regulate Na(+)-glucose cotransport, and there is evidence from animal studies to suggest that the major bulk of sugar is reabsorbed in the convoluted proximal tubule by a low-affinity, high-capacity transporter and that the remainder is absorbed in the straight proximal tubule by a high-affinity, low-capacity transporter. There are at least three different candidates for these human renal Na(+)-glucose cotransporters. This review will focus on the structure-function relationships of these three transporters, SGLT1, SGLT2, and SGLT3.

Partial sequence and expression of the gene for and activity of the sodium glucose transporter in the small intestine of fed, starved and refed chickens

A 970 bp cDNA Na(+)/glucose cotransporter (SGLT1) was isolated and sequenced from chicken jejunum by reverse transcriptase polymerase chain reaction (RT-PCR) using primers based on conserved regions. Using the 970 bp PCR product as a specific probe, Northern Blot hybridization indicated a transcript of ca. 4 kb. The isolated chicken intestinal SGLT1 cDNA was used to quantitate mRNA expression. Glucose uptake activity and kinetics were determined in brush border membrane vesicles (BBMV) from jejunum tissue of chickens which were either fed, food-deprived or refed following food deprivation. Net glucose uptake to BBMV was higher (P: < 0.02) in the control and refed chicks (149 +/- 11.9, 139.6 +/- 7.43 pmol x mg protein(-1) x s(-1)) than in food-deprived chicks (107 +/- 4.23 pmol x mg protein(-1) x s(-1)). The k(m) (150 micromol/L) and V:max (1111.1 pmol x mg protein(-1) x s(-1)) were higher in the food-deprived chicks compared to control and refed birds (25, 24 micromol/L and 227,142 pmol x mg protein(-1) x s(-1), respectively). Expression of SGLT1 mRNA was significantly enhanced in the food-deprived and refed birds. In food-deprived chicks the lower affinity and higher activity of the SGLT1 transporter for glucose were accompanied by higher expression of mRNA which might indicate that the transporter was upregulated by low substrate concentration. Quantification of expression of intestinal mRNA of SGLT1 provides important information concerning control of nutrient uptake.